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13th

World Conference on Earthquake EngineeringVancouver, B.C., Canada

August 1-6, 2004Paper No. 2016

SEISMIC CONTROL OF FEDERAL ELECTRONICS

RESEARCH BUILDING, OTTAWA

Palanimuthu (Ravi) SUNDARARAJ1, R. Tina PALL

21

SUMMARY

The Federal Electronic Research building was designed and built in 1993. In 2003, an additional storeywas added to the existing structure. This building houses very sensitive instrumentation and expensiveequipment. As well, it was of vital importance to the client to safeguard the storage of valuable scientificdata and to provide operational continuity in the event of a major earthquake. This led to the use of aninnovative yet cost effective technology. Introduction of supplemental damping in conjunction withappropriate stiffness was the chosen solution. Incorporating Pall Friction Dampers in steel bracingachieved this.

INTRODUCTION

The Federal Electronic Research building is located in Ottawa, Canadas capital (Figure 1). It wasdesigned by the department of Public Works & Government Services Canada and built in 1993. In 2003,

an additional storey was added. The three-storey building is of concrete frame construction with onebasement. The foundations are on spread footings. This building houses very sensitive instrumentationand expensive equipment. Besides, it was of vital importance to the client to safeguard the storage ofvaluable scientific data and to provide operational continuity in the event of a major earthquake. Theclient expected a higher level of performance than the minimum specified in the building code.

The design criteria stipulated in most building codes, including the National Building Code of Canada(NBCC), are based on the philosophy of designing buildings to resist moderate earthquakes withoutconsiderable damage and to resist larger earthquakes without collapsing. The main emphasis is on lifesafety. In general, reliance for survival is placed on the ductility of the building to dispel energy whileundergoing large inelastic deformations causing bending, twisting and cracking. This assumes permanentdamage, repair costs of which could be economically as significant as the collapse of the building.

In the past, the minimum design provisions of the building codes were adequate for most buildings. Inmodern buildings, circumventing collapse alone is not enough. The cost of finishes, sensitiveinstrumentation and electronically stored data are more valuable than the cost of the structure itself. These

1Public Works & Govt. Services, Ottawa, Canada. E-mail: sundarar@pwgsc.gc.ca2. Pall Dynamics Limited, Montreal, Canada. E-mail: info@palldynamics.com

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must be safeguarded. The problems created by dependence on ductility can be mitigated if a large portionof the seismic energy can be dissipated mechanically, independent of the principal structure.

With the emergence of friction dampers, it has become economically feasible to substantially control thedamage to a structure and its contents. Unlike concrete shearwalls, the friction-damped bracing is notrequired to be vertically over each other. This also gives more freedom in space planning. The plan of the

first storey is shown in Figure 2. The friction dampers have been installed in cross bracing and singlediagonal bracingas required. Architectural planning influenced the location and the type of bracing used.For instance, diagonal bracing allowed for doorways to be easily accommodated. A typical detail of afriction damper in cross bracing is shown in Figure 2. The plan of the building and locations of thefriction-damped braces are shown in Figures 3 and 4, respectively. This building won the OttawaArchitectural Design Prize.

The results of analysis, design procedure, cost benefits and construction details of the chosen structuralsystem will be discussed. A brief discussion on friction dampers is also included so that its applicationcan be better appreciated.

Figure 1. Partial View of Federal Electronic Research Building.

During installation Finished viewFigure 2. Pall Friction Damper in cross bracing.

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Figure 4. Three Dimensional AnalyticalModel.

Friction-damped bracing

Y

Figure 3. Plan View of First Floor, AnalyticalModel.

69.6m

33.7m

X

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PALL FRICTION DAMPERS

Of all the methods available to extract kinetic energy from a moving body, the most widely adopted isundoubtedly the friction brake. It is the most effective, reliable and economical mean to dissipate energy.In late seventies, the principle of friction brake inspired the development of friction dampers, Pall et al. [8-11]. Similar to automobiles, the motion of vibrating building can be slowed down by dissipating energy

in friction. Several types of friction dampers have been developed, Pall et al. [11]. For frame buildings,these are available for tension cross bracing, single diagonal bracing, chevron bracing, and frictionconnectors at expansion joints to avoid pounding.

Pall friction dampers are simple and foolproof in construction and inexpensive in cost. They consist ofseries of steel plates specially treated to develop most reliable friction. The plates are clamped togetherwith high strength steel bolts. Friction dampers are designed not to slip during wind. During severeseismic excitations, friction dampers slip at a predetermined optimum load before yielding occurs in otherstructural members and dissipate a major portion of the seismic energy. Another feature of frictiondamped buildings is that their natural period varies with the amplitude of vibration. Hence thephenomenon of resonance is avoided. After the earthquake, the building returns to its near originalalignment under the spring action of an elastic structure.

These particular friction dampers have successfully gone through rigorous proof testing on shake tables inCanada and the United States. In 1985, a three-storey frame equipped with friction dampers was tested ona shake table at the University of British Columbia, Vancouver, Cherry et al. [3]. Even an earthquakerecord with a peak acceleration of 0.9g did not cause any damage to friction damped braced frame, whilethe conventional frames were severely damaged at lower seismic levels. In 1987, a nine storey three bayframe, equipped with friction dampers, was tested on a shake table at the Earthquake EngineeringResearch Center of the University of California at Berkeley, Kelly et al. [7]. All members of the frictiondamped frame remained elastic for 0.84g acceleration, while the moment-resisting frame would haveyielded at about 0.3g acceleration.

Friction dampers possess large rectangular hysteresis loops, similar to an ideal elasto-plastic behavior,with negligible fade over several cycles of reversals Pall et al. [9], Filiatrault et al. [3]. Unlike viscous orvisco-elastic devices, the performance of friction dampers is independent of temperature and velocity. Fora given force and displacement in a damper, the energy dissipation of friction damper is the largestcompared to other damping devices (Figure 6). Therefore, fewer friction dampers are required to provide agiven amount of supplemental damping. Unlike other devices, the maximum force in a friction damper ispre-defined and remains the same for any future ground motion. Therefore, the design of bracing andconnections is simple and economical. There is nothing to yield and damage, or leak. Thus, they do notneed regular inspection, maintenance, repair or replacement before and after the earthquake. Thesefriction dampers are also compact in design and can be easily hidden within drywall partitions. Thefriction dampers meet high standard of quality control. Before delivery to site, each damper is load testedto ensure proper slip load.

Friction dampers manufactured by Pall Dynamics Limited have found many applications in newconstruction and seismic retrofit of existing buildings, Pall et al. [12-17], Vezina et al. [23], Pasquin et al.[18-20], Godin et al. [4], Savard et al. [21], Wagner et al. [25], Deslaurier et al. [2], Balazic et al. [1], Haleet al. [5,6]. Boeings Commercial Airplane Factory in Everett WA - worlds largest building in volume hasbeen retrofitted with these friction dampers, Vail et al. [24]. Boeing saved more than US$30 million byusing this technology. The City and County of San Francisco chose Pall friction dampers for the seismiccontrol of Moscone Convention Center as it saved them US$2.25 million compared to alternate viscous

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dampers, Sahai et al. [22]. To date, more than eighty buildings have already been built and several areunder design or construction. For more details refer www.palldynamics.com.

NONLINEAR TIME HISTORY DYNAMIC ANALYSIS

The quasi-static design procedure given in the NBCC is ductility based and does not explicitly apply to

friction-damped buildings. However, structural commentary - J of the NBCC, allows the use of frictiondampers for seismic control of buildings. It requires that analysis must show that a building so equippedwill perform equally well in seismic events as the same building designed following the NBCC seismicrequirements. The slippage of friction damper in an elastic brace constitutes nonlinearity. Also, theamount of energy dissipation or equivalent structural damping is proportional to the displacement.Therefore, the design of friction-damped buildings requires the use of nonlinear time-history dynamicanalysis. With these analyses, the time-history response of the structure during and after an earthquakecan be accurately understood.

Three-dimensional nonlinear time history dynamic analyses were carried out using the computer programDRAIN-TABS (Guendelman-Isreal and Powell 1977), developed at the University of California, Berkeley.This program consists of a series of subroutines that carry out a step by step integration of the dynamicequilibrium equations using a constant acceleration in a time step. Although several commercial programsare now available for three-dimensional nonlinear analysis, DRAIN-TABS was the only one available in1992. The modeling of friction dampers is very simple. Since the hysteretic loop of the damper is similarto the rectangular loop of an ideal elasto-plastic material, the slip load of the friction damper can beconsidered as a fictitious yield force.

Different earthquake records give different responses. Three earthquake records, which had peakhorizontal velocity and peak ground acceleration falling within the ranges prescribed by NBCC for Ottawaregion were used. The earthquake record, which gave the maximum response, was an artificial earthquakerecord generated to match the Newark-Blume-Kapur (NBK) response spectra with peak groundacceleration scaled to 0.2g. The analyses were carried out for earthquakes applied along the x-axis, y-axis

and at a 45 degree angle.

Viscous damping of 5% of critical was assumed in the initial elastic stage to account for the presence ofnon-structural elements. Hysteretic damping due to inelastic action of the structural elements and slippingof the friction dampers is automatically taken into account by DRAIN-TABS. Interaction between axial

forces and moments for columns and P- effect were taken into account by including geometric stiffnessbased on axial force under static loads. To account for accidental eccentricity, the center of mass wasshifted by 10% of the building dimension along both axes.

A series of analyses were carried out to determine the optimum slip load of the friction dampers. A totalof 23 friction dampers of 300 kN slip load capacity were required to extract sufficient seismic energy tosafeguard the structure and contents from damage.

The time-histories of deformation in the friction-damped bracing are shown in Figure 5. The maximumslippage in the friction damper is about 13 mm. After the earthquake, the damper nearly returns to itsoriginal alignment under the elastic action of frame. The friction dampers underwent several cycles ofreversals and dissipated a large amount of seismic energy.

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The time-histories of the deflections at the roof level is shown in Figure 6. The maximum drift is 35 mm.The maximum storey drift is about 0.5%, which is about 50% of that specified in NBCC for buildings ofpost disaster importance. At these low drifts, very little damage is expected.

-0.04

-0.02

0

0.02

0.04

0 5 10 15Time, seconds

RoofDisplacement,Y,

-0.015

-0.01

-0.005

0

0.005

0.01

0.015

0 5 10 15Time, seconds

D

ampedBraceDeformation,m

Figure 5. Time Histories of Displacements in Damped Bracing

Figure 6. Time History of Deformations at Roof Level.

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CONCLUSIONS

The analytical studies have shown that the friction-damped structure should perform very well in the eventof a major earthquake. The seismic performance of the structure is far superior to the requirements of thebuilding code. As the seismic forces exerted on the structure are significantly reduced, the system offerssaving in initial construction cost. Besides, the life cycle cost could be significant less as damage to the

building and its content is minimised. The use of friction dampers has shown to provide a practical andeconomical solution for the performance-based design of Federal Electronics Research building.

REFERENCES

1. Balazic, J., Guruswamy, G., Elliot, J., Pall, R., Pall, A. "Seismic Rehabilitation of JusticeHeadquarters Building, Ottawa, Canada", Twelfth World Conference on Earthquake Engineering,Auckland, New Zealand, 2000, Paper No. 2011.

2. Deslaurier, F., Pall, A., Pall, R. "Seismic Rehabilitation of Federal Building, Sherbrooke", CanadianSociety of Civil Engineers Annual Conference, Sherbrooke, 1997, pp.339-348.

3. Filiatrault, A., Cherry, S. (1986) Seismic Tests of Friction-Damped Steel Frames, Third Conference onDynamic Response of Structures, ASCE, Los Angeles, USA.

4. Godin, D., Poirer, R., Pall, R., Pall, A. (1995) Reinforcement Sismique du Nouveau Campus delEcole de Technologie Superieure de Montreal, Seventh Canadian Conference on EarthquakeEngineering, Montreal, 967-974.

5. Hale, T., Tokas, C., Pall, A. (1995) Seismic Retrofit of Elevated Water Tanks at the University ofCalifornia at Davis, Seventh Canadian Conference on Earthquake Engineering, Montreal, 959-966.

6. Hale, T., Pall, R. (2000) Seismic Upgrade of the Freeport Water Reservoir, Sacramento, California,Twelfth World Conference on Earthquake Engineering, Auckland, New Zealand, Paper No. 269.

7. Kelly, J.M., Aiken, I.D., Pall, A.S. (1988) Seismic Response of a Nine-Story Steel Frame withFriction-Damped Cross-Bracing, Report No. UCB / EERC-88/17. Earthquake Engineering ResearchCenter, University of California at Berkeley, 1-7.

8. Pall, A.S., Marsh, C. (1979) Energy Dissipation in Large Panel Structures Using Limited Slip Bolted

Joints, AICAP/CEB Seismic Conference, Rome, Italy, 3: 27-34.9. Pall, A.S., Marsh, C., Fazio, P. (1980) Friction Joints for Seismic Control of Large Panel Structures,Journal of Prestressed Concrete Institute, No. 6, 25:38-61.

10. Pall, A.S., Marsh, C. (1981) Friction-Devices to Control Seismic Response, ASCE/EMD SpecialtyConference on Dynamic Response of Structures, Atlanta, USA, 809-818.

11. Pall, A.S., Marsh, C. (1982) Seismic Response of Friction Damped Braced Frames, Journal ofStructural Division, ASCE, St. 9, 108:1313-1323. (ASCE Raymond Reese Research Prize1983).

12. Pall, A.S., Verganelakis, V., Marsh, C. (1987) Friction-Dampers for Seismic Control of ConcordiaUniversity Library Building, Fifth Canadian Conference on Earthquake Engineering, Ottawa, 191-200.

13. Pall, A.S., Ghorayeb, F., Pall, R. (1991) Friction-Dampers for Rehabilitation of Ecole Polyvalente atSorel, Quebec, Sixth Canadian Conference on Earthquake Engineering, Toronto, 389-396.

14. Pall, A.S., Pall, R. (1993a), Friction-Dampers Used for Seismic Control of New and ExistingBuildings in Canada, ATC 17-1 Seminar on Base Isolation, Passive Energy Dissipation and ActiveControl, San Francisco, USA, 2:675-686.

15. Pall, A., Vezina, S., Proulx, Pall, R. (1993b), Friction-Dampers for Seismic Control of CanadianSpace Agency Headquarters, Journal Earthquake Spectra, Number 3, 9:547-557.

16. Pall, A., Pall, R. (1996) Friction-Dampers for Seismic Control of Buildings A Canadian Experience,Eleventh World Conference on Earthquake Engineering, Acapulco, Mexico, Paper No. 497.

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17. Pall, R., Gauthier, G., Delisle, S., Pall, A. (2000) Friction-Dampers for Seismic Upgrade of QuebecPolice Headquarters, Montreal, Twelfth World Conference on Earthquake Engineering, Auckland,New Zealand, Paper No. 2014.

18. Pasquin, C., Pall, A.S., Pall, R. (1994), Hi-Tech Seismic Rehabilitation of Casino de Montreal, ASCEStructures Congress, Atlanta, USA, 1292-1297.

19. Pasquin, C., Charania, H., Steele, R., Pall, R., Pall, A.S. (1998) Friction-dampers for Seismic Control

of Selkirk Waterfront Offices, Victoria, Sixth U.S. National Conference on Earthquake Engineering,Seattle,USA.

20. Pasquin, C., Leboeuf, N., Pall, R., Pall, A. (1999) Seismic Rehabilitation of Hotel Dieu Hospital,Sainte Hyacinthe, Quebec, Eighth Canadian Conference on Earthquake Engineering, Vancouver, 573-578.

21. Savard, G., Lalancette, J.R., Pall, R., Pall, A. (1995) High Tech Seismic Design of Maison 1 McGill,Montreal, Seventh Canadian Conference on Earthquake Engineering, Montreal, 935-942.

22. Sahai, R., Laws, J., Chen, D., Kong, F., castillo, F., "Performance Based Design of 4-Story MosconeConvention Center Expansion Using Steel Couple Girder Moment Resisting Frame and FrictionDampers", Proceedings, SEAOC Annual Convention, Santa Barbara, California, 2000.

23. Vezina, S., Proulx, P., Pall, R., Pall, A. (1992) Friction-Dampers for Aseismic Design of CanadianSpace Agency, Tenth World Conference on Earthquake Engineering, Madrid, Spain, 4123-4128.

24. Vail, C., Hubbell, J., Structural Upgrade of the Boeing Commercial Airplane Factory at Everett,Washington, Proceedings, Structures Congress & Exposition, May 2003, Seattle, WA, Paper #000529.

25. Wagner, P., Vavak, L., Pall, R., Pall, A., "Seismic Rehabilitation of the New Hamilton Court House",Seventh Canadian Conference on Earthquake Engineering, Montreal, 1995, pp. 951-958.